An analysis of the effects of contextual cues on the development of morphine tolerance in rats.

Tolerance to morphine analgesia was determined daily by exposing rats either to the same box or to different boxes by a repeated administration of morphine (5 mg/kg). In the Acquisition Phase, the rats received either morphine or saline in the same or different boxes for four consecutive days, and the process of tolerance development was assessed by a hot-plate test. Marked tolerance developed in the group exposed to the same contextual cue, whereas tolerance was attenuated in the group exposed to different cues. In the Extinction Phase, all rats received saline injections in the box exposed on Day 1 for four days. On the first day, hyperalgesia was observed only in the rats injected with morphine in the same contextual cue. In the Retest Phase, the rats underwent a second morphine injection and to some extent showed recovery from tolerance. In the Acquisition Phase, the number of animals showing abnormal activity with morphine injection increased monotonically in the group that was administered morphine in the same box (Group M-S) before injection, but in the group administered the drug in different boxes (Group M-D), no systematic development of the activity occurred. These results indicate that the hindering of a rat's ability to associate with environmental cues under the effect of morphine slows the development of tolerance, and the withdrawal and anticipatory symptoms, and the tolerance of morphine involves psychological and pharmacological factors.     

Chlorpromazine hyperalgesia antagonizes clonidine analgesia,but enhances morphine analgesia in rats tested in a hot-water tail-flick paradigm

Seventy-six male Sprague-Dawley rats were tested in a hot-water (55°±0.5° C) tail-flick paradigm. Tail-flick latencies (TFL) were obtained at 30 and 15 min before intraperitoneal injection of either morphine (2.5, 5.0 and 10.0 mg/kg) clonidine (25, 50, 100 and 200 g/kg), chlorpromazine (CPZ, 2.5 and 5.0 mg/kg), dual injections of these drug combinations, or a saline control injection. Further TFL measures were taken immediately following drug administration and thereafter at 15 min intervals. The mean of the pre-drug TFL's served as each rat's baseline. All other TFL's were calculated as percentage changes from that baseline. Mean changes were determined for each treatment group and differences between groups, at each test time, were analysed.Our results demonstrated morphine and clonidine analgesia but CPZ hyperalgesia. The drug interaction studies revealed that morphine analgesia is enhanced by coadministration of either clinidine or CPZ but that clonidine analgesia is antagonized by chlorpromazine. These data suggest that morphine and clonidine exert their analgesic effects through different neurochemical mechanisms. It is particularly interesting that the clonidine-CPZ combination should result in TFL's similar to baseline levels, even though both drugs are sedatives.The investigation emphasizes the value of chlorpromazine as a pharmacological tool in analgesic research because of its ability to induce hyperalgesia even though it is a sedating agent.     

Effects of NMDA receptor channel blockers, dizocilpine and memantine, on the development of opiate analgesic tolerance induced by repeated morphine exposures or social defeats in mice

Development of tolerance to opiates involves various neurochemically and pharmacologically distinct processes. For instance, the diversity of opiate tolerance has been suggested by experiments comparing the establishment of diminished response to different effects of opiate agonists. Antagonists acting at N-methyl-d-aspartate (NMDA) receptors has become a very useful tool for studying opiate tolerance mechanisms since these drugs have been shown to retard the development of tolerance to analgesic properties of opiates. The present study compared the ability of two NMDA receptor channel blockers, dizocilpine and memantine, to affect the development of tolerance to morphine analgesia induced by repeated social defeat or by repeated morphine administrations. Male BALB/c mice were assessed for the tail-flick response before and after the defeat in five social confrontations, or before and after repeated morphine injections (20 mg/kg, s.c., once daily for 8 days). Repeated morphine injections were explicitly paired with environmental cues. Socially-defeated as well as morphine-treated mice developed significant tolerance to morphine analgesia. Separate groups of mice were exposed to repeated social confrontations or injections of morphine with each defeat or morphine injection followed by administration of either dizocilpine (0.03–0.3 mg/kg, i.p.) or low-affinity channel blocker memantine (3–30 mg/kg, i.p.). Both dizocilpine and memantine were effective in preventing the development of repeated morphine-induced tolerance to acute morphine analgesia. Treatments with NMDA receptor antagonists that retarded development of non-associative tolerance also suppressed the establishment of associative tolerance significantly. Social defeat-induced tolerance was prevented by dizocilpine but not by memantine. Our results suggest some degree of similarity in the mechanisms of morphine analgesic tolerance induced by pharmacological, contextual and social stimuli. Received: 13 December 1997 / Accepted: 15 April 1998     

Role of drug-administration cues in the associative control of morphine tolerance in the rat

This research investigated the role of injection procedures as a potential confound in the study of associative and nonassociative morphine tolerance development. Rats administered a series of morphine injections paired with a distinctive context environment can develop tolerance controlled associatively by the context. However, rats given morphine unpaired with the context may also develop some degree of tolerance. This study examined whether this tolerance represents an associative effect with animals using the injection ritual as a cue predictive of morphine delivery. Following 14 days of habituation to handling and injection stimuli, rats were given eight morphine injections (20 mg/kg, IP) explicitly paired or unpaired with a distinctive context. Animals were then tested for morphine analgesia in the context after either a 30-day rest condition or a 30-day period of daily saline injections. Analgesia was assessed by the tail-flick method, and tolerance was defined as the shift to the right of the dose-response curve of morphine-experienced relative to saline control animals. Paired animals across both retention conditions displayed tolerance, whereas tolerance retention in unpaired animals was observed only in those animals not given saline injections over the 30-day interval. Results support an associative interpretation of tolerance observed in unpaired conditions and suggest that the injection ritual may provide highly salient cues for the support of associative tolerance effects.     

A pharmacokinetic-pharmacodynamic model of tolerance to morphine analgesia during infusion in rats

A pharmacokinetic-pharmacodynamic (PK-PD) model was constructed to describe the kinetics of tolerance development to morphine-induced antinociception. Tail-flick latencies in response to hot water (50°C) were assessed in male Sprague-Dawley rats exposed to a 12-hr iv infusion of either morphine (1.4 to 3.0 mg/kg per hr) or saline. Morphine-induced antinociception, expressed as the percentage of maximum possible response (%MPR), peaked after 120 min of infusion and decreased thereafter despite sustained systemic morphine concentrations. Both the rate and extent of tolerance development increased with increasing concentrations; an overall residual effect of approximately 24% MPR was observed at the end of the infusion regardless of the steady-state morphine concentration. The kinetics of tolerance offset were examined in a separate experiment by assessing tail-flick latency 15 min after morphine iv bolus (2 mg/kg) in tolerant and control rats. Recovery of response neared completion 18.5 days after a 12-hr exposure to morphine (2.0 mg/kg per hr). A PK-PD model was constructed to account for the delay in onset of antinociceptive effect and tolerance development relative to the blood concentration-time profile. According to this model, both the extent and the rate of tolerance development were modulated by the kinetics of the drug in the central compartment. Accumulation of a hypothetical “inhibitor” acting either as a reverse agonist, a competitive or noncompetitive antagonist, or a partial agonist could potentially account for the loss of pharmacologic effect in the presence of an agonist. The rate of tolerance development predicted from the PK-PD model varied widely (28-fold) depending on the type of pharmacologic interaction selected to account for the loss of effect. Using the rate of tolerance offset to discriminate between the different models (t_1/2 offset 5.4 days), onset and offset of tolerance was described accurately by postulating that the inhibitor behaves as a partial agonist with low intrinsic activity (5.5% MPR) and high binding affinity for the receptor (IC₅₀ 15.0 ng/ml). Presented in part at the Seventh Annual Meeting of the American Association of Pharmaceutical Scientists, San Antonio, Texas, November 15–19, 1992.     

Discrete cues paired with naloxone-precipitated withdrawal from acute morphine dependence elicit conditioned withdrawal responses

Acute bolus doses of morphine induce a state of acute opioid dependence as measured by naloxone-precipitated withdrawal. Repeated morphine and precipitated withdrawal experience further enhances naloxone-induced withdrawal severity, partly because of direct neuroadaptation to repeated morphine, and partly because of conditioned associations of context and withdrawal experience. To determine whether a discrete tone/light conditioned stimulus could elicit conditioned withdrawal responses in acute dependence, rats trained on a fixed-ratio-15 operant schedule for food reward received morphine (5.6 mg/kg) 4x at daily or weekly intervals, with each morphine injection followed at 4 h by naloxone (1.0 mg/kg) and an operant session. The conditioned stimulus was presented to a Paired group after each naloxone injection. Separate control groups experienced the conditioned stimulus either at a different time of the day or on a different day of the week than naloxone (Unpaired), received naloxone without any conditioned stimulus exposure [Paired-no conditioned stimulus (Paired-NO CS)] or received vehicle instead of naloxone before conditioned stimulus presentation (NaI-Naive). On the test day, all rats received vehicle before conditioned stimulus exposure. The conditioned stimulus alone reliably suppressed responding in Paired groups relative to control conditions with either daily or weekly intervals between conditioning sessions. The administration of morphine 4 h before conditioned stimulus exposure on the test day was not necessary to observe conditioned withdrawal. Thus, conditioned withdrawal is reliably established to discrete cues associated with naloxone-precipitated withdrawal from acute, infrequent (weekly) opioid exposure.     

Effects of short-acting NMDA receptor antagonist MRZ 2/576 on morphine tolerance development in mice

The present study sought to evaluate the ability of a short-acting glycineB site NMDA receptor antagonist, MRZ 2/576, to affect morphine tolerance development in mice. It was found that MRZ 2/576 (10 mg/kg, i.p.) significantly retarded development of morphine analgesic tolerance (20 mg/kg, s.c., 8 days, once a day; tail-flick test) when administered 120 min or 150 min after each daily morphine injection. MRZ 2/576 did not affect the development of morphine tolerance when administered immediately, 15, 30, 60, 90, 180, 240, 300 or 360 min after the daily morphine injections. Thus, short-acting NMDA receptor antagonists may be useful in exploring the temporal characteristics of opioid tolerance (i.e., periods after morphine injection that are critical for tolerance induction) and the present study suggests that after morphine administration there is a period of NMDA receptors activation crucial for the development of tolerance.     

Environmental specificity of tolerance to morphine-induced analgesia in a terrestrial snail: generalization of the behavioral model of tolerance

Terrestrial snails, Cepaea nemoralis, develop tolerance to morphine-induced analgesia, such that after 7-9 days of treatment with morphine (10 mg/kg) their response latencies to an aversive thermal stimuli (38.5 degrees C) are not significantly different from those of untreated control animals. In Experiment A snails were rendered tolerant to morphine using either of two pre-injection cues (light and dark background brightness or color) and then assessed for morphine-induced alterations in thermal nociceptive responses in both environments. In Experiment B snails were made tolerant to morphine in the presence of one of two different thermal cues (a stressful temperature of 35 degrees C that is normally avoided or an ambient temperature of 22 degrees C) and then tested for morphine-induced alterations in nociceptive responses in both environments. In the two experiments tolerance to morphine-induced analgesia was displayed when snails were exposed to the pre-injection environmental cue normally associated with the administration of morphine, but not when exposed to the alternative pre-injection cue. These results demonstrate that various environmental factors (background colors or brightness as well as temperature cues and potentially thermal stress), can function as environmental specific cues for the development of tolerance to morphine-induced analgesia in molluscs, in a manner consistent with a behavioral mechanism of tolerance. Thus, these results suggest that environmental specificity of tolerance involving either classical (Pavlovian) conditioning or habituation may be a general phenomenon having an early evolutionary development and broad phylogenetic continuity.     

On the mechanism of tolerance to morphine-induced Straub tail reaction in mice

The effect of 5-HT and opioid receptor antagonists on morphine-induced Straub tail was studied in mice. Straub tail behavior was induced by subcutaneous administration of different doses (20, 30, and 40 mg/kg) of morphine hydrochloride to mice. The effect of morphine was dose-dependent. Maximum response was obtained with 40 mg/kg of the drug. The response induced by morphine (20 and 40 mg/kg) was decreased by different doses of intraperitoneal injection of naloxone (1 and 2 mg/kg) or methysergide, mianserin, and ritanserin (1 and 2 mg/kg). The effect of morphine (40 mg/kg) was also reduced by intracerebroventricular injection of naloxone (0.4-0.8 microg/animal) or mianserin (2 and 4 microg/animal). Different groups of mice received one daily dose (50 mg/kg sc) of morphine sulfate for 3 days to develop tolerance to morphine. The Straub tail reaction induced by morphine hydrochloride (40 mg/kg) was tested on the fourth day. Naloxone injection (1 and 2 mg/kg ip) on Day 3 (1 h after morphine sulfate injection) or on Day 4 (1 h before test dose of morphine hydrochloride), decreased tolerance induced to morphine. Methysergide, mianserin, or ritanserin (intraperitoneal) on Days 2 and 3 (1 h after morphine sulfate injection) or on Day 4 (1 h before test dose of morphine hydrochloride), also decreased tolerance induced to morphine. Intracerebroventricular injection of either naloxone or mianserin also reduced tolerance to morphine. It is concluded that 5-HT(2) and opioid receptor mechanisms are involved in morphine-induced Straub tail reaction and tolerance induced to morphine also may be mediated through these receptors.     

Morphine and Δ9-tetrahydrocannabinol: Two-way cross tolerance for antinociceptive and heart-rate responses in the rat

Tail-flick analgesic responses and heart-rate changes were measured in male Sprague-Dawley rats challenged with an acute IP morphine sulfate (MS) or ⁹-THC injection after receiving daily injections of ⁹-THC or morphine, respectively. Degree of tolerance development to each agent was determined before the cross-tolerance challenge was administered. Cross tolerance occurred to analgesic and bradycardic effects of a 10 mg/kg THC challenge in rats receiving 50 mg/kg MS injections over a 23-day period. Cross tolerance to the bradycardic effects of a 20 mg/kg MS challenge occurred in rats receiving seven daily 10 mg/kg ⁹-THC injections and to MS tail-flick analgesia after 14 days. Although rapid tolerance occurred during administration of both agents, cross tolerance to THC bradycardia occurred only in groups exhibiting complete tolerance to MS injections; cross tolerance to MS bradycardia was observed in animals that were only partially tolerant to THC injections. The data extend earlier cross tolerance data in the mouse to the rat, and provide new information using heart rate, a response that may mirror aversive internal states induced by drugs.     

The development of morphine antinociceptive tolerance in nitric oxide synthase-deficient mice

Elevations in nitric oxide (NO) have been implicated in the development of morphine antinociceptive tolerance. This study was conducted to establish the role of specific isoforms of NO synthase (NOS) in morphine tolerance development using genetically modified mice. METHODS: Three groups of mice (endothelial NOS [eNOS]-deficient, neuronal NOS [nNOS]-deficient, and NOS-competent) were used in this experiment. On Day 1, the analgesic response (radiant heat tail-flick) to a challenge dose of morphine (4 mg/kg) was determined over 3 hr. Tolerance was induced on Days 1-5 by administering morphine subcutaneously (10 mg/kg) or L-arginine, a NO precursor, intraperitoneally (200 mg/kg), twice daily. Analgesic response to the challenge dose was determined again on Day 6. RESULTS: Following sustained morphine administration, nNOS-deficient mice exhibited less tolerance development when compared to the control group, although measurable tolerance still occurred. Mice deficient in eNOS evidenced a degree of tolerance similar to that of control. Prolonged L-arginine administration produced significant functional tolerance to morphine in NOS-competent and eNOS-deficient mice. The loss of morphine responsivity after L-arginine administration was similar to that after morphine pretreatment. L-Arginine did not affect the antinociceptive response to morphine in mice deficient in nNOS, suggesting that the small degree of morphine-induced tolerance in this group occurs through an alternate pathway. CONCLUSIONS: These data demonstrate the pivotal role of the neuronal isoform of NOS in development of morphine antinociceptive tolerance. Furthermore, tolerance development appears to be predominantly a NO-mediated process, but likely is augmented by a secondary (non-NO) pathway.     

Protective effect of bupropion on morphine tolerance and dependence in mice

Possible reversal of morphine-induced tolerance and dependence by bupropion was studied in mice. A 10-day repeated injection regimen was followed to induce morphine tolerance and dependence. Bupropion (2 and 5 mg/kg) per se, when chronically administered for 9 days, failed to produce any significant change in tail-flick latency compared with the control mice. Chronic administration of bupropion (2 or 5 mg/kg) during the induction phase (days 1-9) delayed the development of tolerance to the antinociceptive action of morphine and also reversed naloxone- (2 mg/kg) precipitated withdrawal jumps. On the other hand, acute administration of bupropion (2 or 5 mg/kg) on day 10, i.e., during the expression phase of morphine dependence, reduced the incidence of naloxone-precipitated withdrawal jumps without affecting tolerance to the analgesic effect. In conclusion, results of the present study suggest the potential use of bupropion in tolerance and dependence.     

The spinal nitric oxide involved in the inhibitory effect of midazolam on morphine-induced analgesia tolerance

Previous studies had shown that pretreatment with midazolam inhibited morphine-induced tolerance and dependence. The present study was to investigate the role of spinal nitric oxide (NO) in the inhibitory effect of midazolam on the development of morphine-induced analgesia tolerance. Subcutaneous injection of 100 mg/kg morphine to mice caused an acute morphine-induced analgesia tolerance model. To develop chronic morphine tolerance in mice, morphine was injected for three consecutive days (10, 20, 50 mg/kg sc on Day 1, 2, 3, respectively). In order to develop chronic tolerance model in rats, 10 mg/kg of morphine was given twice daily at 12 h intervals for 10 days. Midazolam was intraperitoneally injected 30 min prior to administration of morphine. Tail-flick test, hot-plate and formalin test were conducted to assess the nociceptive response. Immunocytochemistry, histochemistry and western blot were performed to determine the effect of midazolam on formalin-induced expression of Fos protein, nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) and nitric oxide synthase (NOS) in chronic morphine-tolerant rats, respectively. The results showed that pretreatment with midazolam significantly inhibited the development of acute and chronic morphine tolerance in mice, which could be partially reversed by intrathecal injection of NO precursor L-arginine (L-Arg). In chronic morphine-tolerant rats, pretreatment with midazolam significantly decreased the formalin-induced expression of Fos and Fos/NADPH-d double-labeled neurons in the contralateral spinal cord and NADPH-d positive neurons in the bilateral spinal cord. Both inducible NOS (iNOS) and neuronal NOS (nNOS) protein levels in the spinal cord were significantly increased after injection of formalin, which could be inhibited by pretreatment with midazolam. The above results suggested that the decrease of the activity and expression of NOS contributed to the inhibitory effect of midazolam on the development of morphine tolerance.     

Involvement of hypothalamic pituitary adrenal axis on the effects of nifedipine in the development of morphine tolerance in rats

It has been shown that nifedipine, as a calcium channel blocker, can attenuate the development of tolerance to the antinociceptive effect of morphine; however, the role of HPA axis on this action has not been elucidated. We examined the effect of nifedipine on morphine analgesic tolerance in intact and adrenalectomized (ADX) rats and on HPA activity induced by morphine. Adult male rats were rendered tolerant to morphine by daily injection of morphine (15 mg/kg i.p.) for 8 days. To determine the effect of nifedipine on the development of morphine tolerance, nifedipine (1, 2 and 5 mg/kg i.p.) was injected concomitant with morphine. The tail-flick test was used to assess the nociceptive threshold, before and 30 min after morphine administration in days 1, 3, 5 and 8. Our results showed that despite the demonstration of tolerance in both ADX and sham operated rats, nifedipine in ADX rats prevented morphine tolerance development at a lower dose (2 mg/kg) than in sham operated rats, however corticosterone replacement prevented nifedipine effect in ADX rats. Acute administration of morphine produced significant increase in plasma corticosterone level, and with repeated injection, a tolerance to this neurosecretory effect was developed. Nifedipine (5 mg/kg) attenuated the acute effect of morphine, but could not block its neurosecretory tolerance.     

Dissociable effects of ultralow-dose naltrexone on tolerance to the antinociceptive and cataleptic effects of morphine

Ultralow-dose opioid antagonists augment the antinociceptive effect of morphine and block the development of tolerance to repeated morphine injections in rodents, but the effects are not reliably reproduced in humans. One explanation for this discrepancy is that preclinical studies of ultralow-dose antagonism in rodents generally use reflex-withdrawal tests of antinociception, which may be affected by cataleptic effects of morphine. We tested this hypothesis by examining whether ultralow-dose naltrexone alters the cataleptic effect of morphine or the development of tolerance to morphine-induced catalepsy. Rats (N=56) were randomly assigned to saline, morphine (10 mg/kg), cotreatments of morphine plus naltrexone (molar ratios of 1,000,000 : 1; 500,000 : 1; 100,000 : 1), or naltrexone-alone groups. Rats were injected with drug for 7 consecutive days; on each day, catalepsy and antinociception were assessed 30 and 60 min postinjection, using the bar and tail-flick tests, respectively. Ultralow-dose naltrexone (500,000 : 1) extended the antinociceptive effect of morphine within a session and attenuated the development of tolerance to the antinociceptive effect of morphine across sessions. Naltrexone alone had no effect on either test. These data show that the paradoxical effect of ultralow-dose naltrexone on antinociception is not the product of morphine-induced catalepsy, pointing to an important role for agonist-antagonist combinations in the clinical treatment of pain.     

Short-term and delayed behavioral effects of pre- and post-weaning morphine in mice

Ninety mouse pups of the CD-1 outbred strain were used to assess activity (Varimex Activity Meter, Columbus Instr., OH) and analgesia (hot plate) after morphine hydrochloride given IP either on days 14-16 (preweanlings) or on days 21-23 (postweanlings). In preweanlings morphine depressed activity already at the lowest dose tested (0.5 mg/kg), and higher doses (1, 5, and 10 mg/kg) did not produce a significantly larger effect. Activity of postweanlings was not depressed until a very high dose (20 mg/kg). By contrast, morphine produced clear analgesic effects at all doses in both preweanlings (day 14) and postweanlings (day 21). Around day 70, activity and hot-plate tests in the no-drug state showed no differences due to prior treatment, except for the fact that hot-plate latencies of mice previously injected with saline as preweanlings were higher than those of all other groups. Twenty-four hr later the tests were repeated after morphine injection (10 mg/kg), and showed a significantly greater depression of activity in mice previously exposed as preweanlings. On the other hand, all groups previously exposed to morphine at either the pre- or the post-weanling stage showed tolerance to the analgesic effect of the drug. These developmental profiles confirm that opioid systems contribute to the modulation of activity by mechanisms which are at least in part separate from those mediating analgesia.     

Context- and cue-conditioned potentiation of acute morphine dependence and withdrawal

Single morphine injections induce a state of acute opioid dependence measured by an increase in naloxone potency to precipitate withdrawal. Repeated morphine exposure (daily/weekly intervals) results in further potentiation of naloxone potency, perhaps due to conditioning mechanisms. The current study tested the hypothesis that previously neutral stimuli could elicit a conditioned potentiation of the withdrawal response following acute bolus injections of morphine. Rats trained on an FR-15 schedule for food received five morphine injections (5.6 mg/kg) at daily intervals. Four hr after morphine injection on Conditioning Days (first 4 days), naloxone (1 mg/kg)-induced suppression of responding was paired either with operant context only, or with a tone/light conditioned stimulus (CS). On Test Day low dose naloxone (0.001-0.33 mg/kg) given 4-h post-morphine preceded the operant session. Rats exposed to naloxone repeatedly in the operant context without CS (Paired-Context Only) showed an increase in naloxone potency on Test Day relative to Unpaired Controls that received all morphine and naloxone in the home cage at a different time of day than operant testing. Rats exposed to the tone/light CS on Conditioning Days also showed a significant increase in naloxone potency relative to Unpaired Controls when the CS was represented on Test Day (Paired-CS), but not when the CS was omitted on the Test Day (Paired-CS/Test Context). Thus, conditioning processes appear to play a significant role in the early development of opioid dependence and withdrawal.     

Early neonatal experience of Long-Evans rats results in long-lasting changes in reactivity to a novel environment and morphine-induced sensitization and tolerance.

In Long-Evans rats, daily 3-h separation from the dam during the neonatal period results in enduring alterations in behavioral and neuroendocrine responses to stressors and sensitivity to antinociceptive effects of acute and chronic morphine. We tested whether early neonatal experience alters sensitivity to effects of morphine on locomotor activity. The subjects were adult rats that had one of the following backgrounds: daily separation from the dam on postnatal days 2-14 for either 3 h (maternal separation (MS)) or 15 min (handled control (H)) or no separation from the dam (non-handled control (NH)). After two consecutive days of baseline activity measurements, subjects were tested daily after SC injections of either morphine (10 mg/kg) or saline for seven days and again on day 10. Beginning five days later, saline and 1.0-10 mg/kg of morphine were tested in all animals. On the baseline days, MS animals had higher horizontal and vertical activity than did NH controls, whereas H animals spent more time in the center of the testing chamber. In MS and H animals but not in NH controls, daily injections of morphine produced progressive increases in all locomotor activity measures, indicative of sensitization (horizontal counts, center time) and tolerance (vertical counts). MS animals with a history of morphine treatment had significantly higher horizontal and vertical activity after a saline injection than did their counterparts with a history of saline treatment, indicative of conditioning. They also exhibited greater locomotor sensitization to 1.0 mg/kg of morphine than did H and NH controls. These results provide further evidence that environmental manipulation in the form of maternal separation early in life results in enduring changes in sensitivity to effects of morphine that could reflect altered endogenous opioid systems.     

The contribution of environmental cues to cross-tolerance between ethanol and pentobarbital

The contribution of Pavlovian conditioning of environmental cues has been studied in relation to tolerance to ethanol-induced hypothermia and cross-tolerance to pentobarbital. Two groups of 12 male Sprague-Dawley rats were exposed every other day to a distinctive set of environmental cues paired with an IP injection of either ethanol 2.5 g/kg or an equivalent volume of isotonic saline. On alternating non-drug days, both groups received saline in the animal room. When they were tested for tolerance to the hypothermic effect of ethanol 2.5 g/kg and cross-tolerance to pentobarbital 25 mg/kg in each environment, tolerance and cross-tolerance in the ethanol-treated group were significantly more pronounced in the ethanol-paired environment than in the saline-paired environment. This indicates the importance of a conditional factor in tolerance and cross-tolerance in this paradigm. Determination of blood levels of ethanol and pentobarbital at various times after injection indicated that conditioned tolerance and cross-tolerance can be explained in part by dispositional factors.     

Effect of number of conditioning trials on the development of associative tolerance to morphine

The acquisition of associative tolerance to the analgesic effects of morphine was investigated by giving independent groups of rats 1, 3, 5, 8, 14, 20, or 30 administrations of drug either explicitly paired or unpaired with a distinctive context. Tolerance, assessed on a tail-flick device using dose-response curve (DRC) methodology, developed more rapidly and reached greater magnitude when morphine and the distinctive context were explicitly paired rather than explicitly unpaired. Tolerance magnitude in both conditions reached a maximum at eight conditioning sessions. It is argued that the tolerance found in both treatment groups was associatively controlled. The function of handling and injection cues as conditioned stimuli, and the deleterious effects of latent inhibition and partial reinforcement on conditioned excitation and conditioned inhibition are discussed.